The invention relates to a method of drawing glass tapes of crystallization-sensitive glass such as borosilicate glass or of glass ceramic. The melt from the vat of a glass furnace is supplied through a feed channel to a drawing chamber from which a glass tape is drawn perpendicularly upwardly and essentially across an entire width of the drawing chamber, and wherein the melt in the drawing chamber is heated.
U.S. Pat. No. 1,615,841, incorporated herein by reference, discloses a method for drawing glass tapes whereby the melt from the melting furnace is supplied through a feed channel having a small cross-section to a drawing chamber from which a glass tape is drawn perpendicularly in an upward direction. The heating of the drawing chamber occurs with two electrodes in the form of metal plates which are disposed at both sides lying opposite the edges of the glass tape. These electrodes extent from the floor of the drawing chamber vat over the entire side wall and extend beyond the edge of the drawing chamber vat. The disadvantage of this heating is that the surface and the remaining melt cannot be differently heated, which is necessary given crystallization-sensitive glasses. A heating in the region of the drawing region along the tape width is not possible since the electrodes are disposed at the end faces. Since only one heating circuit is available, a section-by-section heating is also not possible.
The drawing vat is covered by two component parts which are rigidly disposed at the two sides adjacent to the vertical drawing plane.
The feed of the melt occurs through a non-heatable, closed feed channel which discharges into the drawing chamber below the surface of the melt therein.
British Pat. No. 673,981, incorporated herein by reference, discloses another method wherein a hot zone is generated in the drawing chamber at the glass surface on both sides between the onion and the wall of the drawing chamber. A frame is employed for this purpose, said frame dipping into the melt, extending on both sides parallel to the vertical drawing plane, and comprising surfaces arcing over the surface which are meant to prevent radiant heat losses.
In order to establish these hot zones, at the drawing region there are provided gas or electrical heating units which cannot be differently charged in individual sections. The continuity of the cold film on the glass surface between the drawing region and the wall of the drawing chamber is intended to be interrupted by these two hot zones. A direct heating of the wall of the drawing chamber is not carried out. The drawing chamber can be covered with slides that are adjustable in sections.
It is not possible in this method to heat the drawing chamber wall at the level of the glass surface and therebelow so that, on the one hand, all crystallization is prevented and, on the other hand, the glass is not excessively heated.
U.S. Pat. No. 1,741,886 also discloses that the feed channel can be heated as well. The drawing chamber and feed channel are indirectly heated, i.e. are heated with hot gases at the outside of the glass-carrying, refractory brick.
Usable glass tapes, particularly tapes of borosilicate glass or glass ceramic, cannot be produced with any of the above methods since crystallization at the wall of the drawing chamber lying opposite one another at the vertical drawing plane cannot be prevented.
For example, when processing glass ceramics, the temperature at the point at which the melt separates from the drawing chamber wall cannot drop below a specific value of, for example, 1380.degree. C. because the melt would otherwise crystallize. On the other hand, the melt on its way to the sheet base or root (drawing region) must be cooled to the temperature required for drawing. This can be more than 400.degree. C. lower than the previously mentioned temperature.
It is therefore difficult to simultaneously meet the demands for an adequately high temperature at the point of separation from the wall of the drawing chamber, for a uniform and uninterrupted flow of the melt from the point of separation to the sheet root, and for an adequate cooling over this path to a suitable drawing temperature.